A hybrid vehicle is an eco-friendly vehicle that can reduce exhaust gases produced and improve fuel efficiency by adopting a motor as well as an engine as power sources. The motor and engine are mounted within a power transfer system that separately transfers engine or motor power to a driving wheel, or transfers both engine and motor power to the driving wheel.
A power transmission system for a hybrid vehicle is configured to include an engine and a motor arranged in series with each other, an engine clutch arranged between the engine and the motor to transmit or cut off engine power, an automatic transmission shifting motor power and motor and engine power to a driving wheel and outputting the power, a hybrid starter generator (HSG) (which is a kind of motor connected with a crank pulley of the engine to transmit power to perform engine start and power generation), an inverter that controls the motor and the power generation, and a high-voltage battery connected with the inverter and chargeable and dischargeable to supply power to the motor.
The power transmission system for a hybrid vehicle of the type in which the motor is attached to the automatic transmission is called a transmission mounted electric device (MTED) scheme. The MTED provides driving modes including an electric vehicle (EV) mode, which is a pure electric vehicle mode using only the motor power, a hybrid electric vehicle (HEV) mode using the motor as sub power while using the engine as main power, a regenerative braking (RB) mode collecting braking and inertial energy of the vehicle using power generation in the motor to charge the battery at the time of braking of the vehicle and when the vehicle drives using intertia.
In the HEV mode, the vehicle is driven by the sum of output torques of the engine and the motor simultaneously with lock-up of the engine clutch. In the EV mode, the vehicle is driven only by an output torque of the motor in conjunction with an opening of the engine clutch.
Meanwhile, operating hydraulic pressure of the engine clutch that determines an operation of the engine clutch for transferring and separating power between the motor and the engine can be determined by the initial hydraulic pressure at which torque transfer starts by contacting both ends (an input end and an output end) of the engine clutch and by feedback hydraulic pressure where hydraulic pressure of the engine clutch is controlled by feeding back an engine speed and a motor speed.
In this case, the hydraulic pressure at which the torque transfer starts by contacting both ends of the engine clutch is referred to as a kisspoint.
In the hybrid vehicle, the transfer torque generated in a lock-up state of the engine clutch by contacting both ends of the engine clutch should be known for more accurate transfer torque calculation. An accurate kisspoint of the engine clutch should be known for transfer torque learning.
The kisspoint of the engine clutch is a primary factor used in state determination of the clutch, calculation of the transfer torque, and calculation of initial fill pressure representing hydraulic pressure at the time when both ends of the clutch contact according to the hydraulic pressure of the clutch and varies by a deviation of a single factor and abrasion of the engine clutch. Accordingly, learning for periodically determining an accurate kisspoint time is required for a correct calculation.
In the existing engine clutch kisspoint learning method, pressure at the time when motor torque (output torque of the motor) is varied by gradually increasing the hydraulic pressure of the engine clutch in an open state while a brake is opened by locating a transmission in a parking stage (P stage) or is turned on by locating the transmission in a neutral stage (N stage) is learned at the kisspoint. The learned kisspoint is used in learning the transfer torque of the engine clutch and calculating the transfer torque.
More specifically, in the existing engine clutch kisspoint learning, the learning starts after forcibly placing a transmission in a neutral state while the brake is turned off by locating the transmission in the parking stage (P stage) or the brake is turned on by locating the transmission in the neutral stage (N stage). The hydraulic pressure at the time when the motor torque is varied, by gradually increasing the hydraulic pressure of the engine clutch while the engine clutch is opened while controlling the engine and the motor at different speeds at the parking stage or the neutral stage, is learned with a new kisspoint (see FIG. 4).
FIG. 4 illustrates that the hydraulic pressure at the time when the motor torque is varied is learned with a new kisspoint in kisspoint learning of the engine clutch in the related art.
However, in the existing engine clutch kisspoint learning method, since an entrance condition for the engine clutch kisspoint learning is a situation in which the brake is turned off by locating the transmission in the parking stage (P stage) or a situation in which the brake is turned on by locating the transmission in the neutral stage (N stage), the learning entrance is impossible while driving. As a result, it is difficult to secure sufficient learning frequencies and opportunities.
Further, as the learning frequency of the engine clutch kisspoint is low, when the vehicle starts immediately after ignition, there is a concern that the kisspoint learning will not be performed within a driving cycle until ignition-off from ignition-on of the vehicle.
The above information disclosed in this background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.